The optical trigger on a 240SX may look a bit complicated, with two rows of slits and the unequal slit sizes in the inner ring. However, setting up Megasquirt to work with this ignition is surprisingly simple. Megasquirt does not need or use the information provided by the second ring of 360 slits or the length of the inner slits, so you just need to obtain a signal from the trailing edge of the ring of 4 slits. This writeup covers using a BIP373 for controlling the coil.

Note: DIYAutoTune.com has recently put together a bolt in trigger wheel to allow using this distributor for triggering distributorless ignitions. See this article for more details.

ECU Modifications

It’s choice time, MS-I with MSnS-E firmware, or MS-II. The choice is yours. It’s possible to make your KA run with either ECU. The MS-I with MSnS_E is extremely mature, has a ton of features, and is pretty near the end of its development cycle. The MS-II is much newer, faster hardware, though the development is really just getting kicked off. While it has some features that MSnS_E doesn’t have, the reverse is also true in that MSnS_E has some features that the MS-II doesn’t have (though most require mods to enable them). The writer of the MSnS_E firmware has been conducting some tests on the MS-II firmware and I expect to see some really cool things happen soon…

Using the MS-I PCBv3 with MSnS-E firmware

This is assuming you are starting with an ECU built up like I build my assembled MS130-C MegaSquirt-I PCB3.0 units. If you are building your unit up from a kit you can implement these changes during assembly.

Jumper IGBTOUT to IGN to send to IGBT ignition coil driver signal out of pin 36 on the DB37.

Cut out C12 and C30. (Only needed when triggering from the – terminal of the coil, which you are not)

Remove D1 and jumper it, or just install a jumper wire across it’s leads. (Only needed when triggering from the – terminal of the coil, which you are not)

Cut out R57 if fitted (this won’t be there on my units though).

Get a 330 ohm 1/4w resistor and cut the leads down to about 1/2″ at each end. Maybe a bit less.

Tin each end of the resistor with a bit of solder.

Cut a 5″ piece of hookup wire (22ga is fine) and strip just a 1/8″ or so. Tin the stripped wire with solder.

Melt the tinned wire tip to one end of the tinned 330 ohm resistor tip and let it cool.

Heatshrink wrap this wire/resistor assembly.

Use this wire/resistor combo to jumper the ‘top’ (top as in when you facing the silkscreen side of the PCB, with the text so that you can read it normally) lead of R26 to IGBTIN on the opposite side of the PCB.

That’s it!

Using the MS-II PCBv3 ECU

Build the ECU up to trigger from the Hall/Optical Input (all of our MS230-C units are by default configured this way).

The only change is to enable the IGBT High Current Ignition Coil Driver Circuit to directly drive the coil.

Remove the jumper from JS10 to IGN (if exists)

Jumper IGBTIN to JS10

Jumper IGBTOUT to IGN

(This will enable the BIP373 Ignition Coil Driver)

That’s it!

Wiring

The output is quite straightforward – you will just need to connect the negative terminal of the coil to pin 36. The specifics of the input wiring will depend on which year 240 you have; Nissan changed the wiring colors in 1991. However, the wires serve the same purpose; only the colors changed. Note that these years apply to the 240SX and are not the same for other KA-powered vehicles. For example, it appears that some Nissan pickup trucks with the KA24E engine retained the earlier color scheme for considerably longer.

Distributor Wire Colors

’89-’90

’91 and later

Purpose

B/W

B/W

12v+

G/B

White

MS Trigger (to Pin 24 on DB37, or Relay Board TACH terminal)

G/Y

Black

not used

Black

Black

Ground

Fast idle valves, relays and other components

The 240SX used a thermal type fast idle valve that just needs to be supplied with 12 volt power when the engine is turned on; the fuel pump relay can drive this (and does in Nissan’s factory wiring arrangement). The valve Nissan refers to as the “Air Regulator” will open on its own, so you do not need to set up any sort of idle control with Megasquirt. However, there’s two other fast idle valves on there. One is an on/off valve that opens when the air conditioning is on, which also could be run without the MS. Nissan calls this the FICD valve. There is yet another IAC valve that works as a PWM valve and can be controlled with our MK-PWMIAC mod kit, refered to as the AAC valve.

The stock Nissan ECCS relay is grounded through a circuit in the ECU. Also, the stock S13 wiring harness is set up to send the injectors 12 volts at all times, even when the ignition is turned off. When converting to Megasquirt, the injectors must not have 12 volt power unless the Megasquirt is also powered up. Sending the injectors 12 volt power with the Megasquirt ECU off can result in damage to the flyback circuits. The wiring diagram below shows one way of modifying the stock wiring so that the ECU and injectors power up at the same time, and also allows the other components drawing power from the ECCS relay to work as normal.

An alternate method of dealing with this is to use the Megasquirt relay board instead of the stock ECCS relay to power the Megasquirt, injectors, and crank angle sensor.

The ECU controls the fuel pump relay; taking this over with Megasquirt is simply a matter of taking the wire from the fuel pump relay to the stock ECU, and running it to the Megasquirt’s pin 37.

The gauges operate on their own, except for the tachometer, which is ECU driven.

While the transmission is connected to the stock ECU, this connection only tells the ECU what gear you’re in if you have a manual transmission. The Megasquirt doesn’t need this connection, and the car should work without this just fine.

TunerStudio Configuration for MSnS-E

On the ‘Spark’ menu choose ‘Spark Settings’ and set ‘Spark Output Inverted’ to YES. It is critical to get the Spark Output Inverted setting correct, as setting this wrong can result in damage to the BIP373. Dwell settings will depend on the coil used. A typical rule of thumb is to back off the dwell until you can just barely detect a misfire under lean cruising conditions, then add 0.2 ms of dwell time.

If you use someone else’s .msq file I do recommend you check the MSnS-E firmware version the .msq file was built on as you cannot always use a file built on one version of MSnS_E with a different version– it’s best to use the .msq files with the same version of the MSnS-E firmware that it was built on.

TunerStudio Configuration for MS-II

Important when using the BIP373: On the ‘Ignition Settings’ menu make sure to set ‘Spark Output’ to ‘Going High (Inverted)’ and set ‘Coil Charging Scheme’ to ‘Standard Coil Charge’.

We’d like to hear from you!

These settings cover the basic hardware changes required and some of the TunerStudio settings based on our research and customer feedback, but we are working on some changes to the article based on our experience with the MegaSquirt PNP. We’ll have these changes up soon.

If you have any further information, contributions, or questions about this article, please contact us.